The present invention relates to a photographic lens system and, more particularly, to a high-performance taking lens system suitable for a lens shutter type camera.
In order to realize a high-performance taking lens system for photography, which is capable of displaying superior optical performance as far as the edges of film image field with minimal deterioration of the performance, it is necessary to satisfactorily flatten the sagittal image surface and correct lateral chromatic aberration and comatic aberration.
Incidentally, the basic arrangement of the taking lens system according to the present invention, which will be described later, includes a Tessar type lens and a positive lens added to the image side of the Tessar type lens. Conventional lens systems such as Tessar and triplet type lens systems suffer from the disadvantages that the sagittal image surface tends to have a large curvature and that the astigmatism is also large.
For example, in a lens system such as that disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 55-105216 (1980), the sagittal image surface tends to curve to a considerable extent, and the astigmatic difference increases as the distance from the center of image field increases toward the edge thereof. In addition, the lateral chromatic aberration is not satisfactorily corrected. Therefore, it is difficult to maintain the high performance of the lens system as far as the edge of image field.
Further, many of Tessar and triplet type lens systems have a behind-the-lens stop and hence need to ensure the brightness of the edge of image field and to consider the increase in size of the front lens system.
A lens system such as that disclosed in Japanese Patent Application Post-Exam Publication No. 04-43245 (1992) includes a Tessar type lens and a meniscus negative lens with a convex surface directed toward the image side, which is disposed on the image side of the Tessar type lens. Such a lens system is exceedingly effective in shortening the overall length of the system because the principal point can be disposed forward of the front lens by adding the negative lens on the image side of the Tessar type lens. However, it is necessary in order to realize a reduction in the overall size of the lens system to make the refractive power of the final negative lens relatively strong. Accordingly, the spherical aberration of this negative lens undesirably increases toward the plus side. In general, it is ideal to cancel the spherical aberration produced toward the plus side in the negative lens of the 2-nd lens unit by the cemented lens of the 3-rd lens unit. However, it is unfavorable from the viewpoint of balance to cancel the plus spherical aberration produced in the negative lens of the 2-nd lens unit and the final negative lens by only the cemented lens of the 3-rd lens unit. Therefore, the positive lens of the 1-st lens unit is inevitably assigned the canceling function. Accordingly, off-axis aberrations are also largely affected, as a matter of course. Particularly, coma and astigmatism are adversely affected. Therefore, such a lens arrangement is unsuitable for pursuing high performance, although it is very favorable for reduction in the overall size of the lens system.
Conventional techniques having a lens arrangement similar to that of the present invention have already been known as Japanese Patent Application Post-Exam Publication No. 56-42847 (1981), U.S. Pat. No. 4,240,705, etc. However, all these inventions relate to taking lens systems suitable for single-lens reflex cameras having a long back focus. Therefore, if these techniques are merely applied to a taking lens system for a lens shutter type camera, it is difficult to attain good balance between various aberrations because of the differences in terms of the stop position, the field angle, etc. With regard to the arrangement also, the doublet has a meniscus configuration with a convex surface directed toward the image side, which is different from the doublet in the present invention, which will be described later.